change: Relax requirements from pydantic

Allow using any version from 1.8.2 to 2.0.0 (excluding). This should be
a reasonable balance between stability and flexibility.

setup.py

@@ -26,7 +26,7 @@
     python_requires=">= 3.7",
     packages=find_namespace_packages(where="src", exclude=("test",)),
     package_dir={"": "src"},
-    install_requires=["pydantic==1.8.2"],
+    install_requires=["pydantic >=1.8.2, <2.0.0"],
     extras_require={
         "test": [
             "black",